In hindsight, I never would've cut off my door-shut panels, when only the very bottoms needed repair. It opened up an entire can of worms, that could've otherwise been avoided.
My recommendation would be to cut away only the damaged portion, leaving as much of the original structure alone.
To specifically answer your question about spotwelds, they can more or less be easily drilled out (there IS varying degrees of difficulty, but it's doable). DO NOT settle for cheap knock-off spotweld cutters, only buy the Blair brand. I use two (2) different types on a regular basis; their Rotabroach Series is the preferred cutter (in 5/16 & 3/8" sizes) but requires a 3/8" drill chuck. When working in closer confines, and with a 1/4" RA drill, I use the Blair 3/8" double-ended cutter.
Premium spotweld cutter:
https://www.autobodytoolmart.com/premium-spotweld-cutter-11094-p-11736.aspx
I don't seem to be able to find their smaller one, but the one above usually does most of the work for me. The double-ended, reversible cutter looks like this one:
ALWAYS center-punch the center of the sporweld before trying to drill it out, and lightly touch the cutter to the panel until it's "got its groove" going, or it can walk all over the place. In fact, a few practice attempts on a scrap piece of sheetmetal will pay dividends...
Here's an example of a cleanly removed bracket, by drilling out (only) the spot welds. This piece was later reinstalled, and used the lower three (3) spotweld "remnants" as a reference on where to replace the piece.
And finished!